This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/JP00/00689 which has an International filing date of Feb. 8, 2000, which designated the United States of America.
The present invention relates to an oxidation process which employs an oxidizing catalyst useful for producing alcohols, carbonyl compounds, organic acids, or the like.
Imide compounds such as N-hydroxyphthalimide are known as catalysts used when oxidizing a substrate such as a hydrocarbon, an alcohol, an aldehyde, a ketone, an amine, or a heterocyclic compound by bringing the substrate into contact with molecular oxygen [Japanese Patent Application Laid-Open No. 38909/1996 (JP-A-8-38909), Japanese Patent Application Laid-Open No. 278675/1997 (JP-A-9-278675), Japanese Patent Application Laid-Open No. 327626/1997 (JP-A-9-327626), Japanese Patent Application Laid-Open No. 57814/1998 (JP-A-10-57814)]. An oxidation reaction which employs such catalyst does not particularly require the treatment of exhaust gas, and oxidation products such as alcohols, aldehydes, ketones, and organic acids can be effectively produced under mild conditions at high conversions and selectivities.
However, in a process which employs such catalyst and a co-oxidizing agent, the purity and yield of an object product may sometimes be deteriorated, because by-product(s) difficult to separate from the object compound is generated as a result of deterioration in the activity of the co-oxidizing agent or the oxidizing catalyst (oxidation catalyst). For instance, when cyclohexane, N-hydroxyphthalimide, and cobalt are employed as the substrate, the oxidizing catalyst and the co-oxidizing agent respectively, cyclohexanol, cyclohexanone, and adipic acid are formed. However, when cyclohexanone is produced through a reaction with such reaction system, cobalt as the co-oxidizing agent is poisoned and consequently the catalytic amount of cobalt is substantially reduced. When the catalytic amount of cobalt is small, in the oxidizing reaction step, the formed peroxide is not decomposed sufficiently and therefore a large amount of the peroxide remains in the oxidation reaction mixture. As a result, a product such as the peroxide or an acid derived therefrom by post-treatment is left unseparated in cyclohexanone even after rectification, giving bad influence on the quality of cyclohexanone. Such consequence can be seen not only in the above reaction but also in the case where a catalyst constituted of only the aforementioned imide compound, particularly a catalytic system comprising the imide compound and a co-catalyst is employed in a reaction.
Thus, an object of the present invention is to provide a process which enables the oxidation of a substrate with molecular oxygen without the use of a specific reducing agent while inhibiting the generation of a by-product(s) even when the aforementioned imide compound is employed as an oxidizing catalyst in the reaction.
Another object of the present invention is to provide an oxidation process which makes it possible to smoothly oxidize a substrate while inhibiting the deactivation of an oxidizing catalyst or an oxidizing catalytic system.
Still another object of the present invention is to provide an oxidation process which enables, even under mild conditions, the production of an object compound using molecular oxygen with high purity and in high yield.
A further object of the present invention is to provide an oxidation process which enable the production of, under mild conditions, alcohols, carbonyl compounds, aldehyde compounds, and organic carboxylic acids with high purity and in high yields.
A still further object of the present invention is to provide an oxidation process which enables the production of, under mild conditions, cyclohexanone and cyclohexanol (KA oil), and adipic acid using molecular oxygen with high purity and in high yields.
The inventors of the present invention made intensive and extensive studies to achieve the above objects, and finally found that the water content of the oxidizing reaction system gives a large influence upon the activity of an oxidizing catalyst. The present invention is based on the above findings.
That is, the present invention provide an oxidation process comprising a step of bringing a substrate into contact with molecular oxygen in the presence of a compound having an imide unit (hereinafter, sometimes referred to simply as an imide compound) represented by the following formula (1): 
wherein X stands for an oxygen atom or a hydroxyl group, and the process is characterized in that the water content of thee oxidizing reaction system is not more than 200 mol (for example, about 0 to 50 mol) relative to 1 mol of the imide compound. In the above oxidizing reaction, the imide compound may be a compound represented by the following fomrula (2): 
wherein R1 and R2 are the same or different, each representing a hydrogen atom, a halogen atom, an alkyl group, an aryl group, a cycloalkyl group, a hydroxyl group, an alkoxy group, a carboxyl group, an alkoxycarbonyl group, or an acyl group; R1 and R2 may bond together to form a double bond or an aromatic- or non-aromatic ring; the aromatic- or non-aromatic ring composed of R1 and R2 may have at least one imide unit represented by the aforementioned formula (1); and X has the same meaning as defined above.
R1 and R2 in the formula (2) may be coupled together to form a cyclohexane ring which may have a substituent, a cyclohexene ring which may have a substituent, a 5-norbornene ring which may have a substituent, or a benzene ring which may have a substituent. Moreover, the compound having an imide unit of the formula (1). is employed in combination with a co-oxidizing agent to form an oxidizing catalyst.
In the above oxidation process, the substrate may be at least one member selected from (i) a hydrocarbon having a carbon-hydrogen bond at the allyl site or benzyl site, (ii) a cycloalkane which may have a substituent, (iii) a condensed ring-type hydrocarbon containing a cycloalkane ring or a non-aromatic heterocycle, (iv) a bridged cyclic hydrocarbon containing tertiary carbon, (v) a compound having a hydroxymethyl group at a position adjacent to an unsaturated bond, (vi) an alicyclic alcohol, (vii) an alicyclic alcohol having tertiary carbon, (viii) an aldehyde compound, and (ix) a ketone.
Particularly, the present invention provides an oxidation process useful in producing a cycloalkanone, a cycloalkanol, or a dicarboxylic acid by oxidizing a corresponding cycloalkane which may have a substituent with molecular oxygen.
Moreover, using 0.01 to 0.5 mol of the compound having an imide unit of the formula (1) (if necessary, 0.001 to 0.1 mol of the co-oxidizing agent) relative to 1 mol of a substrate, a water content of the oxidizing reaction system may be maintained of about 0 to 30 mol relative to 1 mol of the compound having an imide unit. Further, a water content of the oxidizing reaction system may be maintained within the above range by removing water produced by the oxidizing reaction from the reaction system. When an oxidizing reaction is conducted in an organic solvent, water resulted from the oxidizing reaction, the solvent and the compound having an imide unit of the formula (1) are separated from the reaction products and the solvent separated is recyclable to the oxidizing reaction system.